Civilian and military sea vessels use acoustic sensor arrays, for example, both active and passive sonar systems for numerous purposes including geological studies, marine life exploration, and military operations such as anti-submarine warfare (ASW). These systems are used to detect the presence of submerged objects by either transmitting a sound wave and detecting its reflection as it propagates through the water (active sonar) or by listening for sound waves generated by these objects (passive sonar).
The use of passive sonar systems may be advantageous over active systems, as passive systems are “silent” in operation. Specifically, a host vessel's location is not reveled by the use of passive sonar systems, whereas the transmission of a sound wave (a “ping”) by an active sonar system, while potentially providing range and bearing information of a target, also greatly increases the ability of other vessels to detect these pings, and thus the presence and/or location of a searching vessel. Accordingly, passive sonar is particularly useful in military operations, such as ASW, where undetected operation is of critical importance.
A drawback of passive sonar, however, is that it is subject to interference, particularly by noise emitted from the host vessel as well as various types of flow-noise, such as turbulent boundary layer (TBL) noise. For example, noise from the vessel's propulsion system may negatively impact the operation of a passive system. This is especially true in the case of hull-mounted arrays, where hull-born vibrations and other noises are transferred directly to the sonar transducers. In order to locate the array further from the vessel's noise-producing components, and thus reduce interference, sonar arrays are often towed behind vessels.
Fiber optic-based acoustic sensors represent promising alternatives to conventional electronic sensors, such as transducer-based hydrophones. Advantages of fiber optic sensors include high sensitivity, large dynamic range, improved channel-to-channel isolation, lightweight and compact size. These optic-based sensors may be particularly useful in towed array sonar systems. In operation, acoustic waves propagating through a medium, such as water, are incident on an optical fiber which results in corresponding changes in length and index of refraction of the fiber. Such environmental changes in turn cause changes in one or more characteristics of the light signal, such as a change in the intensity, phase and/or polarization of a light pulse propagating through the fiber.
Current optical sensors require some form of mechanical device to contain an optical modulating scheme, whether for phase or intensity modulation. Phase devices can be implemented by, for example, a mandrel with an optical fiber wrapped around it, while intensity modulation sensors require mechanical devices to impart some type of mechanical movement to modulate the intensity of light propagating through the fiber. Current optical sensor arrays may also require electronics in the form of demodulation electronics or optical sources contained within the sensor array. The complexity of the array increases by adding these mechanical and electrical devices, resulting in a corresponding increase in cost, and decrease in reliability.
Alternative designs which may reduce cost and/or complexity, as well as increase reliability, are desired.